(1) Field of the Invention
This invention relates to motion transmitting mechanisms and more particularly to flexsplines for harmonic drive type transmissions.
(2) Prior Art
This application defines an improvement on copending, commonly assigned U.S application Ser. No. 07/897,695 now U.S. Pat. No. 4,817,457.
Strain wave gearing generally employs three concentric components to produce high mechanical advantage and speed reduction based on the principle of nonrigid body mechanics. An elliptical wave generator is inserted into a cylindrical strain gear distorting the strain gear into an elliptical form. The strain gear is in the form of a flexible tubular member open at one end with external axially extending teeth around the periphery adjacent the opening. These teeth mesh with the internal axially extending teeth on a ring gear or circular spline. The number of teeth in the ring gear usually exceed by two the number of teeth in the strain gear. Since the teeth on the nonrigid strain gear and the teeth in the rigid circular spline are in continuous engagement along the major axis of the elliptical shape, and since the strain gear has two teeth fewer than the circular spline, one revolution of the wave generator, as the input, causes relative motion between the strain gear and the circular spline equal to two teeth. With the circular spline fixed, the strain gear will rotate in the opposite direction to the input at a reduction ratio equal to the number of teeth on the strain gear divided by two. The strain gear may also be the fixed member and the circular spline would be the output member and rotate in the same direction as the input.
Strain wave gearing was first disclosed in U.S. Pat. No. 2,906,143 granted Sept. 29, 1959. In this disclosure, the cylindrical strain gear was depicted as a toothed member typically welded to a thinner walled tubular portion. The importance of the diaphragm portion was either omitted or grossly shown out of proportion. During the development period of strain wave gearing by the assignee of this patent, a strain gear evolved having three different wall thicknesses. There was a cylindrical portion including an annular gear blank portion having a thickness, in a two lobed configuration, almost always slightly in excess of 2%D where D is the pitch diameter of the strain gear and the remaining tubular portion had a thickness of 0.6%D. Welded to the cylindrical portion to close one end was a plate or diaphragm having a thickness of 0.8%D. The cylindrical portion was made from a cylindrical blank. The blank was machined to have a stepped configuration and a large radius blended the tubular and gear blank portions. The gear teeth were then defined in the annular gear blank.
The tubular portion of the strain gear was made thin to minimize the force to deflect the strain gear, but maintained adequate thickness to transmit the output torque and to facilitate manufacturing. The diaphragm was made thin to minimize the deflection stress associated with the inherent scalloping action yet had adequate thickness to transmit the output torque. The product was improved by increasing the diaphragm thickness to 1% to 2% of D. For details of this construction, see U.S. patent application Ser. No. 06/405,454, filed on Aug. 5, 1982 and now abandoned. Although increasing the thickness increased the deflection stress associated with axial deflection or scalloping, it reduced the stress associated with the axial force acting on it resulting in a more favorable stress condition.
The present invention is an improvement in flexspline mechanisms shown in the prior art as well as that shown in copending U.S. application Ser. No. 07/897,695, incorporated herein by reference.
It is an object of the present invention to manufacture a novel strain gear in a new manufacturing process.